monitoring: add support for custom prometheus rules file

[tarzanek]

since monitoring 4.x you can provide a rule file into prom_rules
directory and it will be picked up

this opens a way for providing custom alerts on top of the provided ones from monitoring bundle
please add support for this

[tarzanek]

I think we need custom name of the file (with some sane default, ev. with a way to disable/enable this):
https://github.com/scylladb/scylla-ansible-roles/blob/master/ansible-scylla-monitoring/defaults/main.yml#L52

the actual cp code can be added after this:
https://github.com/scylladb/scylla-ansible-roles/blob/master/ansible-scylla-monitoring/tasks/common.yml#L151
(perhaps with a name check for collission in file names?)